The present invention relates to a new and improved method of fractionating fiber stock reclaimed from waste paper by means of a sieve device. A cleaned or purified fiber stock suspension freed of heavy and light contaminants or rejects is infed to the sieve device. The fiber stock suspension contains longer and shorter stock fibers which are to be separated from one another.
Additionally, the invention also relates to an installation for the performance of the aforesaid method and contains a stock pulper or slusher for slushing or defiberizing the waste paper, a cleaning device arranged following the slusher and a sorting device.
It is already known in this technology that a sieve device possesses a fractionating effect or action, i.e. is capable of separating longer fibers from shorter fibers of a fiber stock suspension. The shorter fibers preferably penetrate through the sieve, whereas the longer fibers remain at the infeed side of such sieve. Consequently, an infed flow of the fiber stock suspension is divided into two streams. One of these streams effluxes out of a chamber located forwardly of the sieve, the other stream out of a chamber located after the sieve. With the heretofore known methods and apparatuses, both of these streams are however divided in a constant ratio or relationship, and specifically, this is accomplished by regulating one of the streams so as to possess a constant proportion of the infed quantity of the fiber stock suspension. However, with this technique it is not possible to maintain constant the quality of the long fiber proportion and that of the short fiber proportion if the composition of the infed waste paper fluctuates.